Which of the following is the major constituent of the lung surfactant?
**Core Concept**
Lung surfactant is a complex mixture of lipids and proteins that lines the alveoli, reducing surface tension and preventing lung collapse during expiration. It is composed of phospholipids, neutral lipids, and surfactant-associated proteins.
**Why the Correct Answer is Right**
The major constituent of lung surfactant is phospholipid, specifically dipalmitoylphosphatidylcholine (DPPC). DPPC is a type of phosphatidylcholine that is rich in palmitic acid, which allows it to pack tightly at the air-liquid interface in the alveoli, thereby reducing surface tension. This property is crucial for maintaining lung expansion and preventing atelectasis.
**Why Each Wrong Option is Incorrect**
* **Option A:** Surfactant-associated protein (SP) B is a crucial component of lung surfactant, but it is not the major constituent. It plays a key role in regulating surfactant function and stability.
* **Option B:** Neutral lipids, such as cholesterol and triglycerides, are present in lung surfactant but are not the major constituent. They are important for maintaining surfactant function and stability.
* **Option D:** Surfactant protein (SP) A is a crucial component of lung surfactant, but it is not the major constituent. It plays a key role in regulating surfactant function and stability.
**Clinical Pearl / High-Yield Fact**
Lung surfactant deficiency is a critical issue in preterm infants, leading to respiratory distress syndrome (RDS). The use of exogenous surfactant replacement therapy has significantly improved outcomes in these patients.
**Correct Answer:** C. Phospholipid.